A. Field Of The Invention
The present invention relates to a method for cool drying gas.
B. Related Art
More specifically, the invention relates to a method for cool drying gas in which water vapour is condensed from the gas by guiding the gas through the secondary part of a heat exchanger, the primary part of which forms the evaporator of a cooling circuit, which cooling circuit comprises a speed-controlled compressor for the circulation of a coolant in the cooling circuit, as well as a condenser and expansion means, whereby means are applied to determine the evaporator temperature Tevaporator and/or evaporator pressure pevaporator and means for measuring the lowest gas temperature (LAT).
As is known, cool drying is based on the principle that the moisture condenses out of the gas by lowering the gas temperature, after which the condensed water is separated in a liquid separator, and after which the gas is again heated such that this gas is no longer saturated.
Compressed air supplied by a compressor, for example, is in most cases saturated with water vapour, or in other words has a relative humidity of 100%. This means that condensation occurs in the event of a temperature drop to below the so-called dew point. As a result of the condensed water, corrosion will occur in pipes and tools and the equipment can present premature wear.
That is why compressed air is dried, which can be done in the aforementioned way by cool drying. Other gases can also be dried in this way.
When drying compressed air, the air in the heat exchanger must not be cooled by too much as otherwise the condensate could freeze. Typically the dried compressed air has a temperature equal to 20 degrees Celsius below ambient temperature, but never less than two to three degrees Celsius above zero. To this end, the temperature of the coolant in the evaporator is kept between 15° C. and −5° C.
In order to prevent freezing of the condensate, as is known the speed of the compressor is controlled as a function of the measured lowest gas temperature LAT. The LAT is the lowest occurring temperature of the gas to be dried that is guided through the secondary part of the aforementioned heat exchanger.
If the LAT decreases and the condensate threatens to freeze, for example due to a lowering of the gas flow supplied, the speed of the compressor is reduced such that the LAT increases again. The freezing of the condensate is thus avoided.
If the LAT increases, for example due to a rise of the gas flow supplied, the speed of the compressor is increased such that the evaporator temperature falls and the LAT also falls.
A disadvantage of a control based on the LAT is that the evaporator temperature can become too low, such that freezing can occur in the evaporator.
A control based on the evaporator pressure, in other words the pressure in the evaporator, is also known. In such a case the speed of the compressor is controlled such that the evaporator pressure is maintained between certain limits.
A disadvantage of the aforementioned control is that at a low load of the cooling circuit, or for example at a low flow of gas supplied, the condensate can freeze.